Thomas "Moose" Johnson began his organized hockey career in the Montreal City League and at one time played with the junior team on a Friday night, the intermediate team the next afternoon, and the senior team on the Saturday night. After playing two seasons in the Canadian Amateur Hockey League he signed on with the Montreal Wanderers for the Eastern Canada Amateur Hockey Association's inaugural season of 1905-06 where he finished tenth in scoring with 12 goals in a ten-game schedule. In March 1906 the Wanderers ended the three-year reign of the legendary Silver Seven by defeating Ottawa in a two-game challenge for the Stanley Cup.

Prior to the start of the 1906-07 season the ECAHA ruled that professionals would be allowed to play with the amateurs in the league and the Wanderers were quick to sign Johnson, Jack Marshall, Hod Stuart, Pud Glass, and goaltender Riley Hern to lucrative contracts. Those men officially became the first five pro players allowed to compete for the Stanley Cup when the Wanderers successfully won a Cup challenge from New Glasgow in December 1906, prior to the start of the new ECAHA season.

The Wanderers were three games into the 1907 ECAHA season when they hosted the Kenora Thistles in a two-game, total-goals, Stanley Cup challenge series at Montreal. Kenora won both games and took the Cup from the Wanderers by a total score of 12-8, though the Wanderers returned to league play a posted a perfect 10-0-0 season record to qualify to challenge right back.

Johnson continued to impress during the regular season, increasing his goal production to 15 from the previous season and again finishing in the top ten of league scoring. The Wanderers traveled west to Winnipeg in late March 1907 to avenge their earlier loss to the Thistles and reclaimed the Cup in the two-game, total-goal series by a score of 12-8. After the final game a Kenora player cracked Johnson over the head with his stick and cut him for 13 stitches.

Johnson was named to the ECAHA Second All-Star Team in 1908 as he and his Wanderer teammates defended their Stanley Cup title again in January, March, and December of that year, before losing the ECHA title, and thus the Stanley Cup, to Ottawa in 1909. They were champions again in 1910 after winning the newly formed National Hockey Association title from the Ottawa Senators and defended the Cup against Berlin (now Kitchener) on March 12, 1910.

Moose Johnson played one more year for the Wanderers before moving west to play for the New Westminster Royals of the Pacific Coast Hockey Association. He was a perennial all-star in the western league, making the PCHA First All-Star Team in 1912, 1913, 1915, 1916, 1917, 1918, 1919, and 1921. It was while playing in Victoria as a member of the Cougars that he was first called Moose. ""As far as I know,"" he recalled later, ""I was the first 'Moose' in sports history. Now there are dozens."" He was noted for using the longest stick in hockey and had a 99-inch reach. ""The year I quit they buried my stick,"" said Johnson. ""It was the longest stick ever used. In those days there was no size regulations and they couldn't take it from me because it was my livelihood."" His final year in the PCHA was with Victoria in 1921-22.

Johnson played out his career in the minor leagues, making stops in Los Angeles, Minneapolis, Portland, Hollywood, and San Francisco before retiring in 1931.